What can velocity distribution functions of warm protons at comet 67P tell us about infant bow shocks?

The Rosetta spacecraft accompanied comet 67P/Churyumov-Gerasimenko for about two years as the comet approached the sun, passed through perihelion, and headed outwards in the solar system again. The activity level of the comet increased when the distance to the sun decreased and a cometary magnetosphere was born. Different plasma boundaries formed, separating distinct regions, such as the diamagnetic cavity or the solar wind ion cavity. A fully developed bow shock was never observed, as the spacecraft spent most the time close to the nucleus. Instead, measurements revealed what appeared to be the first stages of the formation of shock: the infant bow shock. A characteristic feature of the observations of the infant bow shock is the change of the solar wind proton velocity distribution from fast and cold to slow and warm. Such transitions of the solar wind proton populations have been observed many times at comet 67P.

In this paper we focus on the details of the proton velocity distribution functions during apparent transitions between cold and warm protons. It is not obvious how the distributions look like. Protons that appear warm and slow in an energy-time spectrogram may instead be a (partial) ring distribution. Partial ring distributions were not expected at comet 67P due the small spatial scales but were recently reported. Both solar wind proton and cometary ions seem, at least occasionally, to form partial ring distributions in the cometary environment. We study the shape of the velocity distribution functions during cold proton-warm proton-transitions to confirm or reject the interpretation of the warm protons as an indication of an infant bow shock. We also try to understand the physics behind the change of the velocity distribution and, if applicable, the formation of the infant shock.